1. A. M. Savchuk and A. A. Shkalikov, Mat. Zametki 66(6), 897 (1999) [Math. Notes 66 (6), 741 (1999)].
2. A. M. Savchuk and A. A. Shkalikov, Trudy Moskov. Mat. Obshch. 64, 159 (2003) [Trans. Moscow Math. Soc. 64, 143 (2003)].
3. Ph. Hartman, Ordinary Differential Equations (New York, 1964; Mir, Moscow, 1970).
4. N. N. Konechnaya, Mat. Zametki [Math. Notes] (in press).
5. M. A. Naimark, Linear Differential Operators (Nauka, Moscow, 1969) [in Russian].